This invention relates to a method for producing a semiconductor device and more particularly to a method for forming a stack type capacitor.
There is known in the prior art a semiconductor device with a stack type capacitor, for example, a dynamic random access memory (DRAM). The stack type capacitor comprises a lower electrode of a storage node formed on an interlayer dielectric layer, a capacitor dielectric film of a metal oxide such as tantalum oxide (Ta.sub.2 O.sub.5) formed on the storage node, and an upper plate electrode formed on the capacitor dielectric film.
In order to increase a capacitance of the stack type capacitor, it is known in the art to provide side walls of polycrystalline silicon or poly-silicon on the side surfaces of the storage node.
JP-A-03-16552 discloses a method for forming the storage node with the side walls. In the method, a poly-silicon layer for the storage node is formed on the interlayer dielectric layer having a contact hole. The interlayer dielectric layer consists of three films of a silicon oxide film, a silicon nitride film and a silicon oxide film. On the poly-silicon layer, a silicon protective film is also formed, and the poly-silicon layer and the silicon protective film are selectively etched to form the storage node with the protective film on the storage node. Then, a poly-silicon layer is formed on the protective layer and the side surfaces of the storage node. Then, the poly-silicon layer is subjected to the anisotropic etching and is partially remained on the side surfaces as side walls. Thereafter, the protective oxide film on the storage node is etched out.
Thus, the storage node with the side walls are formed. On the storage node and the side walls as the lower electrode, the metal oxide film such as tantalum oxide film is formed as the capacitor dielectric film, and the upper plate electrode is formed on the metal oxide film. Thus, the stack type capacitor is formed in the semiconductor device.
However, the method has a problem described below. When the protective film on the storage node is removed after formation of the side walls, the uppermost silicon oxide film of the interlayer dielectric layer is also partially removed so that a cavity is formed to extend to the underside of the storage node. When a titanium nitride film is formed as the upper plate electrode by the sputtering method on the tantalum oxide as the capacitor dielectric film, the titanium nitride film cannot be formed in the cavity because the sputtering method is poor in the step coverage. Accordingly, the stack type capacitor does not have good properties.
In order to solve the problem, it is proposed to use a silicon oxide film and a silicon nitride film as the protective film and the uppermost film of the interlayer dielectric layer, respectively. According to the proposal, although the cavity is not formed, the silicon nitride film exists as the interlayer dielectric layer of the semiconductor device. This results in a different problem that the semiconductor device suffers from the isolation deterioration and increase of leakage, as is pointed out by Mori et al in a paper entitled "Analysis of Isolation Degradation Induced by Interlayer Material in Capacitor Over-Bit-Line (COB) DRAM Cell" in Extended Abstracts of the 1994 International Conference on Solid State Devices and Materials, Yokahama, 1994, pp. 904-906.